[BUG] AzMon OTel log exporter incorrectly serializes log bodies

[lmolkova]

Log bodies could have complex structure, but exporter calls str on them

azure-sdk-for-python/sdk/monitor/azure-monitor-opentelemetry-exporter/azure/monitor/opentelemetry/exporter/export/logs/_exporter.py

Line 147 in d3cfd06

name=str(log_record.body)[:32768],

As a result, they are not a valid json ({'content': 'tell me a joke'}) and are unparseable in AzMon